1. Field of the Invention
The present invention relates to a display device including organic EL (Electro Luminescence) elements or the like, and an electronic device having such a display device.
2. Description of Related Art
In a field of display devices for image display, a display device using current-drive optical elements as light emitting elements, for example, a display device using organic EL elements (organic EL display device) has been recently developed and is being commercialized, the current-drive optical element being changed in emission luminance in accordance with a value of electric current flowing into the optical element.
The organic EL element is a self-luminous element unlike a liquid crystal element or the like. Therefore, the organic EL display device does not need a light source (backlight), and therefore high in image visibility, low in power consumption, and high in element response speed compared with a liquid crystal display device that needs a light source.
A drive method of the organic EL display device includes simple (passive) matrix drive and active matrix drive as in the liquid crystal display device. In the simple matrix drive, while a device structure is simplified, a large display with high resolution is inconveniently hardly achieved. Therefore, the active matrix drive is being actively developed at present. In the active matrix drive, electric current flowing into an organic EL element disposed for each pixel is controlled by an active element (typically TFT (Thin Film Transistor)) in a pixel circuit provided for each organic EL element.
In such an organic EL display device, a current-voltage (I-V) characteristic of the organic EL element degrades with the lapse of time (temporal degradation) as well known. In a pixel circuit that current-drives the organic EL element, when the I-V characteristic of the organic EL element is changed with time, a value of current flowing into a drive transistor is changed. Thus, a value of current flowing into the organic EL element is also changed, and accordingly emission luminance is changed.
In the organic EL display device, each pixel is typically configured of three sub-pixels corresponding to three primary colors, R (red), G (green) and B (blue), or four sub-pixels including a sub-pixel corresponding to a color of W (white) in addition to the three sub-pixels. In this case, as well known, rate of the degradation of the organic EL element is different for each of individual-color sub-pixels, and thus temporal color shift occurs in each pixel, leading to reduction in display image quality.
A reason for such difference in degradation for each of individual-color sub-pixels mainly includes a fact that a characteristic (luminous efficiency) of a luminescent material of an organic EL element is different for each of colors. As another reason, density of current (current density) flowing into the organic EL element is different for each of individual-color sub-pixels to adjust white balance. This is because current density needs to be set high in a sub-pixel corresponding to a color, where luminous efficiency of the organic EL element is relatively low, compared with in sub-pixels of other colors, leading to increase in degradation rate of the relevant sub-pixel.
Thus, for example, the following two methods are proposed to suppress temporal color shift caused by the latter reason (difference in current density). In the first method, an aperture ratio is varied for each of individual-color sub-pixels, thereby while current density is not varied for each of colors unlike the above, degradation rate is equalized between colors (for example, see Japanese Unexamined Patent Application Publication No. 2006-215559). In the second method, a plurality of sub-pixels are provided for one color in each pixel, thereby while current density is not varied for each of colors, degradation rate is equalized between colors as in the first method (for example, see Japanese Unexamined Patent Application Publication No. 2004-311440).